Ordovician Unit, Oslo Region

ELNES FORMATION

(Elnesformasjonen)

(New name)

(Formerly termed: 4a-α 1-4 , Ogygiaskifer, Upper Didymograptus Shale, Ogygiocaris Shale, Cephalopod Shale).

Main Lithology. Shale (Elnes Shale, Elnesskiferen).

Basal Stratotype. Oslo-Asker: Bjørkåsholmen (Fig. 11) (NM 836 286).

Thickness Variation. The tectonic incompetence of this formation compared to that of the underlying and succeeding units has resulted in its deformation in most districts by faulting and folding. Thus even in the best exposed area, (Oslo-Asker) its thickness can only be estimated (ca 60m). In the less well exposed districts no reliable estimate is possible other than to note that in Modum it may be in the region of 120m thick although this may include faulted repetitions of parts of the sequence.

Definition. The Elnes Formation was named without definition by Harper (1986) and Owen (1987). It is a dominantly shale unit the basal beds of which have previously been regarded as transitional from the limestones of the Huk Formation. In the environs of Oslo, this transition is represented by dolomitic limestones, while further west in Asker and also the other districts of the region a bedded limestone-shale unit (the Helskjer Member) is developed. In both cases the boundary with the underlying Huk Formation is sharp and in all but Skien-Langesund, Ringsaker and west Toten the top of that formation is characteristically rich in nautiloid cephalopods. In Oslo-Asker the Elnes Formation is divided into four members. A local member (the Heggen Member) is recognised in the southern districts but from Ringerike northwards the formation remains undifferentiated above the basal, Helskjer Member.

Høy & Bjørlykke (1980) introduced the term Bjørge Formation without formal definition on the Hamar map sheet in the Nes-Hamar district. The Norwegian Stratigraphical Committee have informed us that this formation name for the units previously termed the 'Ogygiocaris Series' has no formal status and hence the Elnes Formation is defined in Oslo-Asker and the name applied in the Mjøsa areas. Here, and in Hadeland, the succession above the Helskjer Member comprises black shales with limestone nodules, many of which weather to a rusty brown colour and although nodular limestone bands are common in the lower few metres of the member they became progressively rarer. In the middle part of the succession the shales are generally up to 40cm thick and the limestone nodules rarely more than 5cm in thickness although the upper part of the section at Hvattum in Hadeland also shows 20cm thick shale horizons separating 8cm limestone nodules. These may be equivalent to the nodular limestone described as marking the top of the 'Ogygiocaris Shale' in the Mjøsa area (Holtedahl 1909 p.8, Skjeseth 1963 p.64). The uppermost part of the Elnes Formation shows the shales becoming thicker and limestones rarer. In the Mjøsa districts the nodules of this part of the member are more rounded in outline than the flat nodules lower down and also contain barytes Størmer 1953 p.103; Skjeseth 1963 p.64). They contain abundant cephalopods and hence this part of the succession was termed the 'Cephalopod Shale' (Holtedahl 1909, Størmer 1953). It is not possible even to estimate the total thickness of the Elnes Formation in the northern part of the Oslo Region.

Fauna and Age. As is discussed below, the members of the Elnes Formation contain a diverse shelly and graptolitic fauna. Berry (1964) concluded that the graptolites indicated a range from the base of the Didymograptus murchisoni Zone to the top of the Glyptograptus teretiusculus Zone and thus a middle Llanvirn to early Llandeilo age. An historical account of work on the graptolites of the Elnes Formation was also given by Berry (1964 pp.64-68) and need not be repeated here. The basal Helskjer Member, however, contains a trilobite fauna which is very close to that of the Megistaspis gigas Zone in Sweden, which is latest Kunda in age and thus equivalent to the uppermost part of the D. 'bifidus' graptolite Zone (Wandås 1984). The naming of this lower Llanvirn graptolite zone has been problematical following the recognition that D. bifidus is an Arenig species from Canada and is not conspecific with the British Llanvirn material traditionally ascribed to it. Fortey & Owens (1987 p.90) have suggested that D. artus become the eponymous species for the lower Llanvirn biozone in Britain, although the base of such a zone has yet to be formally defined.